1. Field of the Invention
The invention relates to removal and treatment of dissolved phase contaminant plumes from groundwater, and in particular the treatment of large-scale groundwater plumes.
2. Description of the Related Art
A contaminant plume comprises a head, the source of the contamination; a tail, the down-gradient end of the contamination; and a body, the dissolved phase contamination between the source area and the leading edge of the plume.
Common approaches to treating or managing a contaminant plume in groundwater focus on treatment or management of the head or the tail of the plume.
Other existing systems treat or manage the body of the plume. These existing systems generally entail some form of containment accomplished through groundwater pumping, to prevent the contaminant plume from migrating further down-gradient.
Some existing systems for treating the body of the plume use biological and/or chemical treatments, and generally endeavor to either bring the plume to a reagent, or deliver reagent to the plume. Approaches to bring the contaminated water to the reagent typically involve either pumping, amending (e.g., mixing with reagent), and reinjecting; or pumping with some form of ex-situ treatment.
Approaches to deliver the reagent to the plume typically involve: 1) direct injection in a grid pattern over the area of the plume; 2) placement of reagent in a trench or injecting less mobile reagent (e.g., emplacement of low solubility (slow release) substrate, zero valent iron, etc.) transverse to the plume at the down-gradient edge (i.e., a permeable reactive barrier); 3) injection of soluble reagent (i.e., soluble substrate, etc.); or 4) some form of recirculation.
For large plumes, some form of recirculation is generally required to facilitate delivery of the treatment reagent over increasingly large distances. Recirculation may be accomplished longitudinally through delivery of reagent in a recirculation pattern in line with the direction of groundwater advection, or in a transverse pattern through delivery of reagent in a recirculation pattern perpendicular with the direction of groundwater advection.
Existing recirculation systems require a network of injection and extraction wells throughout the entire surface area above the contaminant plume. The spacing between injection and extraction wells is dependent on the ability to establish an engineered hydraulic gradient that results from the combined groundwater mounding (around injection wells) and groundwater drawdown (around extraction wells). Under the best hydrogeologic conditions, existing recirculation systems have a maximum distance of 200 to 300 feet between injection and associated extraction wells. At distances greater than these, recirculation does not adequately decontaminate a plume.
It may be impractical or impossible to locate extraction wells at distances of less than 300 feet from the associated injection well, for example, a large contaminant plume may be under a surface area that may have commercial structures, roads, homes or other existing infrastructure that prevents or interferes with placement of injection and extraction wells at distances less than 300 feet.
Recirculation can also be technically impractical because the natural hydraulic gradient at a site may overcome the attempted recirculation. Large plumes continue to move over time, and existing methods cannot treat large water volumes rapidly enough to overcome the natural water movement. In other instances, the length of time needed to complete the recirculation is too long to be practical.
Other existing systems rely on a natural hydraulic gradient to cause the needed flow for water treatment,
Some existing systems use a biological reagent to treat or decontaminate the water, for example, systems where extracted contaminated water is mixed with a biological reagent and then reinjected. These systems are subject to biofouling.
Biofouling occurs when bacteria attach, grow and block the equipment, piping well screen, filter pack or formation surrounding a nutrient delivery well, thereby limiting or preventing the proper function of the process piping, equipment, or wells. The bacteria may originate in the aquifer itself, or may be introduced during reagent addition, or groundwater recirculation. Most groundwater environments contain an active and diverse microbial population, but growth is limited until substrate is introduced.
Upon introduction of substrate to support bioremediation, biological waste may accumulate from the enhancement of indigenous bacteria. Biofouling will often accumulate and restrict pipes, pumps or other apparatus that come in contact with the contaminated water and treatment bacteria.
As plume areas increases in size the surface area that is above a contaminated plume also increases in size, and existing surface conditions may interfere the placement of treatment wells. Existing systems are inefficient or ineffective for the treatment or management of large-scale plumes. Computer modeling showed that existing pump and treat, or recirculation systems failed to effectively treat large-scale contaminant plumes, due to cost, the natural hydraulic gradient or other factors.